The photo shows a message demanding money on a monitor of a payment terminal at a branch of Ukraine's state-owned bank Oschadbank in Kiev on June 27, 2017. (Photo by Reuters)

Ukraine has been targeted by the worst cyber attack in its history after a ransomware caused outages on computer networks of the country’s government and other key organizations.

Prime Minister Volodymyr Groysman described the Tuesday cyber attacks as “unprecedented,” but noted that the virus failed to target "important" IT systems in the country.

"It was an unprecedented attack, but our IT experts are doing their work and protecting strategic infrastructure. Important systems have not been affected," the Ukrainian premier said in a post on Facebook.

Secretary of Ukraine’s National Security Council Oleksandr Turchynov said there are signs of Russian involvement in the attack.

However, Russia's top oil producer Rosneft was also another victim of a similar hacking. The company said on Tuesday that a large-scale cyber attack targeted its servers, noting that its oil production was unaffected.

According to Ukrainian Interior minister advisor Anton Gerashchenko, Cryptolocker virus, a version of WannaCry ransomware which attacked 200,000 computers in more than 150 countries in May, targeted the country’s IT systems.

"The ultimate goal of the cyber attack was to try to destabilize," he wrote in a Facebook post, noting that the attacks were probably launched from Russia.

"We also have a network 'down'," Rozenko said on Facebook. "This image is being displayed by all computers of the government," he said.

Ukrainian Prime Minister Volodymyr Groysman

Ukrainian Deputy Prime Minister Pavlo Rozenko said the government had a network “down” with “all computers of the government” displaying an error message.

The cyber attack also hit banks, the state energy distributor and Kiev's main airport.

Ukraine's central bank warned financial institutions to enhance security measures after the cyber attack hit several banks in the country, hindering their operations.

"All the financial market participants have taken steps to tighten security measures to counteract these hacker attacks," it said, adding that "banking infrastructure is securely protected" and further attacks "will be efficiently warded off."

Oschadbank, one of Ukraine's largest banks, restricted services to its clients after it was affected by the ransomware.

Earlier in the day, Kyivenergo power company in Kiev said it was forced to turn off “all of our computers” after its network was attacked.

Ukraine's delivery service company Nova Poshta was also attacked by Petya.A ransomware which locks users out of the system and demands purchase of a key to resume access.

Kiev's main airport, Boryspil, was another target of the spam attack which may cause some flights to be delayed.

"In connection with the irregular situation, some flight delays are possible," Boryspil Director Yevhen Dykhne said on Facebook.

In May, hackers attacked hundreds of thousands of computer systems across the world. The virus encrypted data on the computer systems and displayed messages on monitors demanding a payment of $300 in the almost untraceable virtual currency Bitcoin.

The screen messages warned that the payment must be made within three days, otherwise the price would be doubled; and if none is received within seven days, the locked files will be deleted.

According to Microsoft President Brad Smith, US intelligence agencies, such as the CIA and National Security Agency (NSA), were to blame for the attack because they stockpiled malicious software code which was used by the hackers.